1. Field of the Invention
The present invention relates to a biodegradable poly(lactic acid) having improved physical properties and a process for its preparation. More particularly, the present invention relates to a simple and economically efficient process for preparing a poly(lactic acid) having a high molecular weight in high yields which comprises direct polycondensation of lactic acid using a polyhydroxyl compound having at least four hydroxyl groups, and to a biodegradable poly(lactic acid) of a star-shaped molecular structure with a high molecular weight prepared by this process.
2. Description of the Prior Art
Lactic acid found in nature is nontoxic and harmless to animals, plants, and human beings. Poly(lactic acid)s, which are polymers of lactic acid, can be easily hydrolyzed in the presence of moisture. Due to these properties, poly(lactic acid)s have been used in biodegradable medical applications such as suturing threads or staples for surgery, sustained release polymers for drug delivery, etc., as well as in agricultural applications for soil treatment including herbicides, soil disinfectants, and the like.
In recent years, poly(lactic acid)s have attracted commercial interest for solving environmental pollution problems caused by plastic wastes. The application of poly(lactic acid)s have been extended for use as general-purpose biodegradable polymeric materials for packaging materials, food containers, coating materials, and so forth. The polymeric materials for these uses must have a high molecular weight in order to provide the desired strength to the resulting products.
A number of the processes for preparing a poly(lactic acid) having high molecular weights have been known in the art. For example, Japanese Patent Publication No. (Sho) 56-14,688 discloses a process for preparing a poly(lactic acid) which comprises condensating lactic acid to give an oligomer; pyrolizing the oligomer in the presence of a catalyst such as antimony oxide, antimony fluoride, stannous chloride, etc. to give a cyclic diester, and ring-open polymerizing the resulting diester in the presence of a catalyst such as stannous octoate, diethyl zinc, etc. However this process is complicated, and time and labor consumptive.
U.S. Pat. No. 4,273,920 discloses a process for preparing a poly(lactic acid) having a molecular weight of 6,000 to 30,000 which comprises polycondensation of lactic acid in the presence of a highly acidic ion exchange resin such as Dowex 50W (available from Dow Co.). One of the defects of this process is that a quantity of the solid catalyst (Dowex 50W) contained in the resulting polymer cannot be removed by any industrial purification techniques. In addition, the resulting polymer tends to be colored by the ion exchange resin used.
Japanese Laid-Open Patent Publication No. (Sho) 62-64823 describes a process for preparing a poly(lactic acid) having a molecular weight of about 10,000 which comprises polycondensating lactic acid in the presence of a fluid paraffin. This process is relatively simpler than that mentioned above, but it has a defect in that the resulting polymer has a low molecular weight and thus, shows poor physical properties which are not useful as a general-purpose biodegradable material.